RNA from both normal cells and cells transformed by Moloney sarcoma virus (MSV) have been analysed for the expression of viral specific sequences. The MSV genome contains sequences derived from Moloney leukemia virus (MLV) and a cellular host sequence (src). The latter sequence is responsible for transformation by MSV. Recombinant DNA bacteriophage lambda clones containing either the integrated provirus of two MSV isolates or the normal mouse cell sarc sequence from Balb/c mice have been used as probes in detecting the expression of genomic and subgenomic messenger RNAs in a variety of infected and uninfected cell lines. A direct comparison of the nucleotide sequence of the viral src and host sarc is in progress. This information in conjunction with the analysis of the representative messenger RNAs transcribed from these genes will be used in determining the genetic elements required for transformation.